Resistors with high resistance values are often used in the implementation of analog circuitry. Providing resistors with high resistance values was not a problem in earlier technologies as high sheet resistance resistors were available from the standard process flow. However, with the scaling down of critical dimensions in state of the art CMOS technologies high sheet resistance resistors are typically not available. For example, in a typical 0.18 micron technology, the poly resistors generated from a conventional process flow have a low sheet resistance ranging from 200 to 400 ohms per square. In order to obtain high sheet resistance resistors, additional process steps and mask levels are required. This is undesirable as it adds to the complexity and cost of fabricating the ICs.
As a result, in current advanced deep sub-micron (DSM) processes, analog designers either use low sheet resistance resistors to implement high value resistances or make do with smaller resistance values. The former approach, however, is not efficient for the miniaturisation as large resistors which consume more silicon space are required in order to implement high value resistances. FIG. 2 illustrates a traditional gain step circuit (see Grebene “Bipolar and MOS Analog Integrated Circuit Design”, John Wiley, New York, 1984, FIG. 7.3, p. 313 and also see Gray et al., “Analysis and Design of Analog Integrated Circuits”, 2nd edition, John Wiley, 1984, FIG. 6.3, p. 354) wherein the gain of the circuit is given by:
  Gain  =                    V        0                    V        i              =          20      ⁢                        log          10                ⁡                  (                                                    R                202                            +                              R                203                                                    R              201                                )                    
Since the gain of the circuit is dependent on the ratio of the feedback resistors (R2 202 and R3 203) over input resistor R1 201, a large feedback resistor value is needed in order to obtain a high gain. For example, assuming that the values of the resistors in FIG. 2 are normalised and input resistor R1 201 has a resistance of unit value, the feedback resistor value will have to be 16 times larger in order to obtain a 24 dB gain. For the embodiment shown in FIG. 2, the large feedback resistance value is implemented by the large resistor R2 202 in combination with the smaller resistor R3 203. When using the circuit 200, the switch S0 is always closed. The large resistor R2 202 is selected by closing the switch S1 221 and opening the switch S2 222 in order to obtain the 24 dB gain. In order to obtain 0 dB gain, the switch S1 221 is open while the switch 222 is closed, thereby bypassing the large resistor R2 202. The resistor R3 203 is shared between both gain steps in order to reduce the total resistance value. The resulting large resistor area for resistor 202 not only consumes more silicon area, but also degrades analog matching performance, especially in high gain stages where the gain accuracy depends on the matching between big resistors (e.g. 202) and small resistors (e.g. 201 and 203). In such situations, the typical solution is to implement the small resistor using a parallel combination of large resistors. However, this not only increases the area further, but also leads to an increase in power consumption (if the resistor value is decreased to minimize the area increase) and possible degradation of the overall performance.
Another prior-art solution is to use the popular R-2R network 300 shown in FIG. 3 (see Grebene, FIG. 14.5, p. 759). For the example illustrated in FIG. 3, there are four branches 301. Each of the branches has a sub-branch connected to a virtual ground to produce a 6 dB gain variation for each branch. The four branches 301 are switched on or off together to produce a 0 or 24 db gain. Selective switching of the branches can be realized by a set or reset S1 control bit. Switches 320 are always turned on when the gain circuit is in use.
As evidenced from the above discussion, it would be desirable to have a resistor efficient gain circuit which reduces resistor area without adversely effecting the functionality of the circuit. Additionally, for some applications it is also important to have the gain circuit input impedance remaining constant so as not to change the external ac coupling network frequency response of the circuit and also avoid external input source loading effects.